1mL of yeast slurry is about 1 billion cells. That is of course assuming the sediment is pure yeast! In a re-pitch scenario, about 25% of the slurry is yeast, and the other 75% is proteins, dead yeast, hops and other junk, so you want to count 4mL as 1 billion cells. After washing, you can count on 2mL as 1 billion cells. Assuming it is freshly harvested! Yeast mortality rate is about 50% after only two weeks so re-pitch right away.
From there, count on pitching 100 million cells per liter per degree plato of wort for ales, or 200 million cells per liter per degree plato for lagers. In a typical 5 gallon batch of 1.060 wort, this would be about 500mL of washed yeast (after decanting).